1. Field of Invention
The invention relates to an automatic document feeder having a sheet-lifting mechanism, and more particularly to an automatic document feeder which can lift up a leading edge of a sheet elastically to facilitate a pick-up roller in picking up the sheet and a sheet feeding roller in feeding the sheet subsequently.
2. Related Art
Automatic document feeders are widely used in the apparatuses such as scanners, printers, copiers and multi-function peripherals. An automatic document feeder using a pick-up roller to pick up a sheet is commonly used in conjunction with a paper tray having a larger capacity.
Due to the mechanical design of the pick-up roller and for collocating the pick-up roller and the large paper tray, the large paper tray is usually provided with a stopper for stopping the leading edges of a stack of sheets disposed in the paper tray. So, the sheet, which is picked up by the pick-up roller, has to pass over the top of the stopper so as to reach a separating roller installed in the automatic document feeder smoothly. The stopper may be designed to have a steep or vertical surface for restricting the position of the leading edge of the sheet. In this case, when there are many sheets stacked in the paper tray, the topmost sheet of the stack is very close to the top of the stopper, and the topmost sheet can pass over the stopper without any difficulty. However, when few sheets are left in the paper tray, the topmost sheet is farther from the top of the stopper, and it is difficult for the sheet to pass over the stopper, thereby causing some disruption in the sheet feeding process.
In order to solve this problem, the stopper may be designed to have a slightly inclined surface such that the slope of the stopper is smaller. In this case, the user may unintentionally insert the sheets into the clearance between the separating roller and the sheet separating pad, and the sheet-separating function may be disabled. In addition, when there are few sheets in the paper tray, the topmost sheet still can pass over the top of the stopper easily because the slope of the stopper is gradual, despite that the topmost sheet is farther from the top of the stopper. When many sheets are disposed in the paper tray, some upper sheets are very near the top of the stopper. So, multiple sheets may easily pass over the stopper such that the stopper cannot function to stop the sheet and the sheet cannot be easily fed. In addition, the gradually sloped stopper occupies larger space because it has the inclined surface with the smaller slope. Thus, the prior art cannot solve the problems presented when the sheets in the paper tray are many or few.